Omnidirectional magnetic particle flaw detecting apparatus

ABSTRACT

An omnidirectional magnetic particle flaw detecting apparatus is disclosed. The apparatus includes two sets of magnetizing coils crossed in such a manner that an elliptical rotating magnetic field may be formed with its major axis running along the width direction of test material at the center of the crossed coils at the time when said coils are air-cored, and a circular rotating magnetic field may be formed on the surface part of the test material at the time when such material is inserted into the coils as the material progresses in the lengthwise direction.

United States Patent 1191 Domon et al. 1 Apr. 8, 1975 OMNIDIRECTIONALMAGNETIC 2.338.793 1/1944 Zuschlag 324/34 R PARTICLE FLAW DETECTING3,354,385 ll/l967 Wood et al. APPARATUS 3,495.l66 2/1970 Lorenzi et al324/37 [75] Inventors: Hitoshi Domon; Takaharu Yasuda, O EIG ATENTS 0R APLICATIONS both of Kitekyttshu; Katsuhiro 847,661 6/1952 Germany 324/37Kawashirna, Fukuoka, all of Japan [73] Assignee: Nippon SteelCorporation, Tokyo, Primary Examiner-Robert J. Corcoran Japan Attorney,Agent, or Firm-Wenderoth, Lind & Ponack [22] Filed: July 16, 1973 [21]Appl. No.: 379,336 [57] ABSTRACT An omnidirectional magnetic particleflaw detecting [30] Foreign Application Priority. Data apparatus isdisclosed. The apparatus includes two sets July 20. 1972 Ja an 47-72716of magnetizing coils crossed Such a manner that an Aug. 8, 1972 Ja an47-92970 elliptical rotating magnetic held y be formed With June 13.1973 Ja an 48-66680 its major axis running along the Width direetieh oftest material at the center of the crossed coils at the time 52 us. c1324/38; 324/37 when Said eehe ere eh-eeted, end e eheulet rotating 15111m. 01. 001p 33/12 megnetie held y be formed on the surfeee P of [58]Field of Search 324/37. 38, 40, 41 the test materiel at the time WhenSueh material is serted into the coils as the material progresses in the5 References Cited lengthwise direction.

UNITED STATES PATENTS 8 Claims, 13 Drawing Figures 1.129.584 2/1915Murphy 324/37 mEMEsAPR 19. 5 3.878832 SHEET 1 0F 5 FIG. 3

A a A m 5 2' con. TURN RATIO aITEIIITs-IPII 3,876,932

SIIZET 2 [IF 5 MAJOR AXIS- MINOR AXIS RATlO- N 0-! J U1 (D 30 60 90 I2'oI5IO COIL CROSSING ANGLE (gouss) FIG. 5

LEAKAGE FLUX FROM A SLIT (AS {FLAW) LYING PERPENDICULARLY TO THEDIRECTION OF PROGRESS x {LEAKAGE FLUX FROM A SL|T(AS FLAW) LYING AT 45-AGAINST THE DIRECTION OF PROGRESS LEAKAGE FLUX FROM A SLITIAS FLAWILYINGPARALLEL WITH THE DIRECTION OF PROGRESS -80 -60 -40 -2o 0 20 4'0 60 sbmmFLAW DETECTING POSITION sum 3 OF 5 FIG. 6

c b Cl FIG. 7

[I a b l9 FIG. 8

(gouss) {LEAKAGE FLUX FROM A SLIT (AS FLAW) u LYING PERPENDICULARLY TOTHE 50- DIRECTION OF PROGRESS x LEAKAGE FLUX FROM A SLIT (AS FLAW) LYINGAT 45 AGAINST THE DIRECTION OF PROGRESS LEAKAGE FLUX FROM A SLIT(ASFLAW)LYING PARALLEL WITH THE DIRECTION OF PROGRESS 8O -o 40 -20 6 2'04'0 6'0 8 0mm FLAW DETECTING POSITION DETECTING SENSITIVITY NPATENTEDAPR 81975 3 876 932 sninsu g FIG. I2

, 1 a OMNIDIRECTIONAL MAGNETIC PARTICLE FLAW DETECTING APPARATUSBACKGROUNDOF THE INvENTIoN 1. Field of the Invention The presentinvention relates to amagnetic particle testing apparatus for flawdetectionof magnetizied articles and, more particularly, to anomnidirectional magnetic particle flaw detecting apparatus which is ableto detect flaws regardless of their directionrelative-to the device sothat flaws in a test material lying in a direction perpendicular to the,direction of travel of the test material can be detected with the samelevel ofdetecting sensitivity as flaws lying along the direction oftravel of test material.

2. Description of they Prior Art.

In the case of flaw detection of magnetized steel or :the, like, it isdifficult to detect flaws lying along the.di- 1 rection oftravel(lengthwise) of the ,material; under scrutiny while-detecting flawslying in the direction perpendicular to the direction oftravel since themagnetizing direction must be perpendicular to the direction alongwhichflaws lie.-I n such case, therefore, different magnetizing units forflaw detecting are used in the lengthwise and widthwise directionsv oftest material. Thus, according to.the conventionalamethods for magneticparticle flaw detection,.at least two different units are required,making operationscomplicated,and its automation very difficult in viewof the necessity of taking into-considerationthe relationships betweenmagnetizing units and between the test material and each respectiveunit.

As, one of the ,improvements made to the, conventionalmagnetic particleflaw detecting apparatus in order to solve the. above-mentioned problem,so-called .crossed coils, which cross each other at right angles,

are used as magnetizing coils. This system utilizes current withphases'different by' 1205' ina three-phase current source. However, thissystem, to say nothing of the conventional apparatus, has not been;successful in solving the problem ?of detecting flaws lying in variousdirections, all in thesame level of detecting sensitivity.

SUMMARY OF THE INVENTION It is an object oftthe present invention toprovide a magnetic particle flaw detecting apparatus, which is veryhandy, and which can detect flaws'jlying in various directions,-all innearly the same level of detecting sensitivity.

vAnother objectof the present invention is to provide a magneticparticle flawdetecting apparatuswhich can detect flaws at any positionon-the surface away from the intersection of coils: that is,,which canperform omnidirectional detection.

A further object of the present invention is to provide a magneticparticle flaw detecting apparatus which is so efficient that it candetect flaws at the, same time all over the entire width of wide, plainmaterials, such as slab or thick plate, in all directions, withoutcontacting flaws.

These and further objects of the present invention lar rotating magneticfield should be formed on the surface of the test materials. However, incase magnetizing coils, which are to form a circular rotating magneticfield when no test material is positioned therein, are

used for flaw} detection of materials having a length greater than thewidth, and particularly in case such materials are inserted into thecoils along theirlengthwise direction, the magne tic field produced onthe test material in the lengthwise direction extends to a much greaterextent than that in the widthwise direction, as the material itself hasbeen magnetized. This results in the reduction of the magnetic fieldproduced in the widthwise direction relative t o'the lengthwisedirection,

making it impossible to conduct tests with constant detectingsensitivity in every direction over the test material. I

As a result of efforts made by the inventors of the present invention,it was found that omnidirectional constant sensitivity flaw detection ismade possible by BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is aperspective view of magnetizing coils of the flaw detecting apparatusaccording to the present invention.

. FIG. 2 shows a circuit for the connection of the electric source withthe coils of FIG. 1.

FIG. 3 shows the relationship between coil turn ratio and the majoraxis-minor axis ratio of the elliptical rotating-magnetic field formedat'the center of the coils, when these coils are air-cored.

FIG. 4 shows the relationship between the angles formed by crossingmagnetizing coils and the major axis-minor axis ratios of the ellipticalrotating magnetic field formed at the center of the coils, when thesecoils are air-cored.

FIG. 5 shows the result of flaw detection achieved by the flaw detectingapparatus of the present invention. FIG. 6 shows the slit flaws inbillets used for the test .of FIG. 5.

I FIG. 7 is a front view of a billet as it is inserted into amagnetizing coil.

FIG. 8 shows the result of flaw detection carried out by a prior artflaw detecting apparatus.

' FIG. 9 is a perspective view of the apparatus of the present inventionhaving two sets of crossing magnetizing coils.

' FIG. 10 is a schematic illustration of a device for switching theelectric source between coils for use in the apparatus of FIG. 9.

FIG..11 is a perspective view of magnetizing coils .used in theembodiments of the present invention.

. FIG. 12 is a perspective view of a magnetizing com- ,posite coil madeof the magnetizing coils of FIG. 11.

FIG. 13 shows an electric power circuit connected with the magnetizingcomposite coil of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, amagnetizing coil 1 is constructed with a first composite coil 2 and asecond composite coil 3.

Thefirst composite coil 2 has a rectangular shape, and has an outer coil4 and an inner coil 5. The outer coil 4 and inner coil 5 are wound onthe same reel, but have different numbers of turns.

The second composite coil 3 is constructed in the same manner as thefirst composite coil 2. The two composite coils 2 and 3 are arranged tocross each other at the centers respectively of their correspondinghorizontal parts, thus forming the magnetizing coil 1.

FIG. 2 shows the connection between the electric sources respectivelyfor the first composite coil 2 and the second composite coil 3.

In FIGS. 1- and-2, numerals 8 and 9 denote terminals of the outer coil 4of the first composite coil 2 for the connectionof an electric source;numerals 10 and 11 denote terminals of the inner coil 5 for theconnection of the electric source; numerals l2 and 13 denote terminalsof the outer coil 6 of the second coil composite 3 for the connection ofthe electric source; and numerals 1 4 and 15 denote terminals of theinner coil 7 for the connection of the electric source. Letters u, v,and w of three-phase current source 17 denote phases for the outputterminals on the secondary side of the transformer which is subjected toV wire connection in the three-phase electric source.

Alternating current is supplied to the outer coil 4 of the firstcomposite coil 2 from the terminals 8 and 9, connected with thealternating current source 17; and an alternating current with a certainphase difference, in this case 120 from the phase of the currentsupplied to saidouter coil 4, is supplied to the inner coil 5 in thereverse direction to that to the outer coil 4. The same descriptionapplies toalternating current supplied to the second composite coil 3.

The coil turn ratio between the outer and the inner coils of respectivecomposite coils is determined by considering the shape of the testmaterial, the phase difference between alternating current supplied tothe coils and other factors, so that a nearly circular rotating magneticfield may be formed on the surface of the test material during the flawdetecting process.

The crossing angle 0 between the composite coils 2 and 3, defined in theregion through which the test materials travel, is so determined that acircular rotating magnetic field may be formed on the surface of thetest material.

The shape of the composite coils is not limited to a rectangle, but mayvary appropriately according to the sectional form of test materials,and in each composite coil the two coils may be placed side by sideinstead of the above-mentioned superposition of one coil over the other.7 i

The following is a detailed explanation of the magnetic particle flawdetecting apparatus according to the present invention. As shown in FIG.2, alternating current having a phase difference of 120 relative to eachother, is supplied to the coils 4 and 5 of the first composite coil 2.The same thing applies to the coils 6 and 7 of the second composite coil3. Also, as shown in FIG. 2, the relation between the first compositecoil 2 and the second composite coil 3 is maintained through theirconnection with the electric sources.

The coil turn ratio between the outer coil 4 and the inner coil 5 of thefirst composite coil 2 is preferably between 1.5 and 2.5. The same ratioalso applies to the second composite coil 3. Outer coils 4 and 6 andinner coils 5 and 7 have, respectively, the same number of turns.

If the coil turn ratio is less than 1.5, the major axisminor axis ratioof an elliptical rotating magnetic field will greatly increase, but thisis not practical, since this requires a large amount of power to attainthe required detecting sensitivity. On the other hand, if the coil turnratio is more than 2.5, the major axis-minor axis ratio will approach\/3 which situation will appear as if the major axis and the minor axis ofthe elliptical rotating magnetic field were reversed in comparison withthe air-cored arrangement when the material is in the flaw detectingtest, thereby causing a difference in detecting sensitivity betweentests in the width and in the length of the test material, which doesnot meet the purpose of the present invention.

FIG. 3 shows the relationship between the abovementioned coil turn ratioand the ratio between the major axis and the minor axis of theelliptical rotating magnetic field at the center when the coils areaircored; if the coil turn ratio is less than 1.5, the major axis-minoraxis ratio will gradually approach infinity as the ratio approaches 1;and as the coil turn ratio becomes greater, the major axis-minor axisratio gradually approachesv 3.

As mentioned above, the desired rotating magnetic field can be obtainedby determining the appropriate coil turn ratio according to the presentinvention. Further, for readily adjusting the ratio between the majoraxis and the minor axis of the elliptical rotating magnetic field to thedesired value, the composite coils having the above-mentioned coil turnratio, are made to cross each other, but it is then necessary to makethe crossing angle 0 acute. In case this angle 0 is an obtuse angle,that is, more than it will be impossible to obtain an ellipticalrotating magnetic field having its major axis running along the width ofthe test material when the coils are air-cored.

, According to the knowledge and experience of the inventors of thepresent invention, an angle 0 of less than 60 is effective to obtain anoptimum elliptical rotating magnetic field; if the angle 0 is too small,the effective width of the passage of material will be narrowed, makingit necessary to increase the size of the magnetizing coils. FIG. 4 showsthe relationship between the crossing angle of coils on one hand and theratio between the major axis and the minor axis of the ellipticalrotating magnetic field at the center of the coils on the other hand. Itis apparent that the optimum major axis-minor axis ratio can be obtainedfrom a crossing angle between 60 and 30. Detection signals generated bythe magnetic flaw detecting apparatus of the present invention areprocessed in a known circuit and are output as flaw detecting signals.

In connection with the above explanation of the construction of theapparatus of the present invention, the following is an example of flawdetection, using the apparatus of the present invention.

Magnetic flaw detection (the determination of leakage flux) wasconducted with a flaw detecting apparatus constructed with magneticcoils, each having a coil turn ratio of 4 :'2 obtained by making thenumber of actual turns of the outer coil and of the inner coil,respectively,"4 and 2. The composite coils are then crossed,

four magnetic particle sprayers'19, one being directed against each sideof the billet, as is well known in the art. The results of themeasurement of leakage flux, for quantitative confirmation of theefficacy of the subject invention, are shown in FIG. 5. Asis obvioustherefrom, all the slits a, b, and c were detected as flaws with nearlythe same detecting sensitivity even though slit a was perpendicular tothe direction of travel A of the billet of rolling billets; slit b wasat 45. to this direction and slit was parallel with said; direction.This clearly establishes that all slits can'beldetected with nearly thesame detecting sensitivity. FIG. 8 shows the result of a flaw detectiontest which was made on a material of the same kind and size as mentionedabove, that. is, a square steel billet, with a prior art apparatusconstructed with two coils, each coil having six turns and crossed atright angles. As seen in the figure, the apparatus does not overcome theprior art difficulties specified in the instant application and achievespoor results compared with the test obtained with the subject invention.More.particularly, the level of detecting sensitivity foraslit lyingperpendicular to and 'parallelwith the direction of progress isreversedbetween FIG. 8 and FIG. so far as the detecting position is inthe vicinity of the center of the in tersection of the magnetizingcoils. This proves that the apparatus ofthe present invention isefficient; it works with, nearly the same level of detecting sensitivityregardless of the directions of slits (flaws) made on the apparatus maywork with low detecting sensitivity.

FIG. 9 is a, perspective view of a magnetic particle flaw detectingapparatus designed toobviate this disadvantage. Magnetizing coil 21consists of composite coils 23 and 24, constructedas detailed above, andis superposed with similarly constructed magnetizing coil 22 consistingof composite coils 26 and 27. The device .may be coupled to a powersource in a manner similar .to that detailed above. The coils arepositioned such that an imaginary lineconnectingthe' points ofintersection 25 of the composite coils 23, 24 and an imaginary lineconnecting the points of intersection 28 of the "composite coils 26, 27cross each other at right angles .and at the. respective center of theimaginary lines.

If the angle of vintersection formed by the imaginary line s deviatesfrom 90, the rotating magnetic field is deformed, making uniformlyeffective detection impossible. As forjniaterials'having such sectionalform as of shaped steel, the'angle should be changed according to thesection form. a

The apparatus of the present invention is constructed as mentionedabove, and shall be operated as follows:

A test material, such as a square steel billet, is introv duced into thestructure built as shown in FIG. 9 in the direction of the arrowHowever, current should not be supplied simultaneously to themagnetizing coils 21 and 22; this introduction would create a mixture ofrotating magnetic fields formed by the respective coils 21 and 22 whicheffects the range of the flaw detection. In order to avoid thiscomplication, the apparatus of the present invention is so operated thatthe current supply is alternated at a high rate between the coils 21 and22, so that they alternately and singly operate for a limited time. Thismethod of current supply will not cause mixing of the ellipticalrotating magnetic fields produced by respective coils,- making itpossible to carry out flaw detection on amulti-face basis.

FIG. 10 shows an embodiment of a device for switching the powersourcefor the above-mentioned purpose. This device is so constructedthat a thyristor 32 and a step-down transformer 33 are connected inseries, thereby alternating the current from the source be tween thecoils 21 and 22. If the rate of travel of the test materials isassumedto be 60 m/min. (l in/sec.) and an alternating current of 60 Hzis switched every 2 cycles (1/30 sec.), the material will progress 6.7cm during ,present invention, to supply a small amount of current,

say, less than 10 percent of the normal supply, since no substantialrotating magnetic field isformed thereby.

Thus, flaw detection of squarebillets over the entire surface area canbe made in one pass through the apparatus of the present invention withconstancy in every direction and with a high level of detectingsensitivity.

The operation of the magnetic particleflaw detecting apparatus fortesting broad articles like plate, is explained, as follows: In tliiscase, the composite f'coils made of two sets of magnetizing coils arenot suff cient to cover the breadth of such material. Thereforefamagnetizing coil series made of more than two sets of magnetizing coilsconnected in series at the intersections thereof are used.

However, in such a case, magnetic fields interfere witheach other at theconjunction of the intersections of coils in a series, so that themagnetic fields in the widthwise direction are cancelled, leaving thoseof lessenedlevel'of sensitivity only in the direction of travel.

The crossed coils shown in FIG. 12 may be used to solve theabove-mentioned problem.

FIGQll shows a magnetizing element coil 41,'which isniadefby' crossingtwo coils 42 and 43 and forminga certain crossing angle Obetween theirrespective top parts 44 and 47' and also between their respectivebottorn partsj45' and 48. Numerals 46 and 49 indicate sides,respectively, of the coils 42 and 43. The crossing angle 0 and thecurrent phase difference between the c'oils42 and 43 are left to choicein view of the above disclosure. Tabs 73, 74, and 76 indicate terminalsfor connecting the coils with an electric power source. According to thepresent invention, more than two of such unit magnetizing element coils,as shown in FIG. 12, may be connected in series into two conjunctivemagnetizing coil series. Two such series 51 and 52 may be superposed sothat the intersection 56 of the magnetizing element coils of the secondconjunctive magnetizing coil series 52 is placed at the center of theopening 55 between the connecting parts 53 and 54 of the magnetizingelement coils of the first conjunctive magnetizing coil series 51.

Thus, conjunctive magnetizing composite coils are formed havingchain-like intersections at the top and thebottom parts. Superpositioncan be made in any ent invention. Also, according to the presentinvention,

the number of. magnetizing element coils may be adjustedin accordancewith the width of the test materials. The intersection 56 of the secondconjunctive magnetizing coil series needs not be strictly the center ofthe opening 55 between the intersection 57 of the first conjunctivemagnetizing coil series, but may be in the vicinity of the center. Themagnetizing coils are serially coupled at their top and bottom portions,respectively.

At the center, that is, the boundary of the intersections 57 of theadjoining conjunctive magnetizing coil series, the magneticfieldsproduced at the intersection 56 interfere with each other tocancel the magnetic field in the direction of connecting magnetizingelement coils, that is, in the width direction B, leaving a weakenedfield in the direction of travel A. I

Therefore, it is impossible to have flaw detecting on an omnidirectionalbasis carried out at a position far away from the above-mentionedboundary part, that is, the intersections of magnetizing element coils.In order to solve this problem, the first and the second conjunctivemagnetizing coil series are superposed, as mentioned above, and thecurrent supply is switched alternately between these series, thuspreventing simultaneous supply. Numerals 61,62, 63, 64, and 65 and 66indicate terminals of respective conjunctive magnetizing coil series forthe connection with their electric power source. Each of these numeralsis used for two 7 terminals in order to point out the connectionpositions 'to ,t he current source output terminals shown in FIG. .13, q

The magnetic particle flaw detecting apparatus of the present invention,which is constructed as mentioned above, is operated in the followingmanner. A plate-like magnetic article, such as thick steel plate, ischarged into the space formed by the conjunctive magneticcoil series 51and 52 along the direction line A, and when it is in the air-core of theconjunctive magnetic composite coil for flaw detection, if it is sodevised as to have. the magnetic field produced on the surface of thetest material form a circular rotating magnetic field, anomnidirectional magnetic field will periodically travel over the fullwidth of the test material, making. it possible to conduct flawdetection with the same level of detecting sensitivity on anomnidirectional basis I In flaw detection of plate-like magneticarticles: by using the apparatus of the present invention, a strongermagnetic field is produced in the direction of travel A i of testmaterials, as such materials have already been magnetized from outside.However, the strengthening travel)-of an elliptical rotating magneticfield to be between 1 and 2 at the time the conjunctive composite coreis empty. In order to determine an appropriate value in the rangebetween 1 and 2, such various factors as current phase difference andthe crossing angle of crossed coils are considered; the most usefulphase difference being 120.

In this case, the crossing angle 0 of magnetizing element coils ispreferably between 120 and FIG. 13 shows a circuit for connecting flawdetection coils 51 and 52 of the apparatus of the, present invention tothe current source.

In the figure, numerals 67, 68 and 69 indicate periodical currentswitches, such as 32 in FIG. 10. In the up position of the switches,only conjunctive series 51 is energized. In the down position, onlyconjunctive series 52 is energized. Numerals 70 and 71 denote respectivetransformers having V-connection with the three-phase current source.Letters U, V, W, u, v and w denote, respectively: the primary and thesecondary phases of the terminals of the transformer. Letters R, S and Tdenote the phases of lines of the three-phase current source.

The inventors of the present invention were successful inomnidirectional flaw detection over the entire surface of thick steelplate test materials by using the magnetic particle flaw detectingapparatus constructed with the connection of magnetizing element coilsinter 'secting at right angles, as shown in FIG. 12, and by using athree-phase alternating current having a phase difference of n Thisbeing the achievement of the apparatus of the present invention, it isnow possible to subject broad plate-shaped magnetic articles to flawdetection on an omnidirectional basis over the whole surface areawithout contacting materials at high accuracy and efficiency.

What is claimed is:

1. An omnidirectional magnetic particle flaw detecting apparatus,comprising a first set of magnetizing coils consisting of a first planarcomposite coil and a second planar composite coil the planes positionedto intersect each other, each said composite coil being composed of twocoils sharing one coil reel but having different numbers of coil turns,the coil turn ratio between the two coils of each coil composite beingso determined as to produce 'a nearly circular rotating magnetic fieldon the surface of a test material when the test material is insertedinto the magnetizing coils, and an alternating current source meanscoupled to the two coils of each of said composite coils and supplyingalternating current respectively differing in phase. relative to the twocoils, whereby surface flaws can be detected with the same sensitivityindependent of the flaw direction.

2. The apparatus claimed in claim 1, wherein the angle formed by theintersection of the planes of the first composite coil and the secondcomposite coil in the direction in which the test material is insertedis acute.

5. The device of claim 1, further comprising a second set of magnetizingcoils consisting of a third planar composite coil and a fourth planarcomposite coil, the planes of said third and' fourth composite coilsbeing positioned to intersect each other and being positionedconcentrically with said first and second composite coils such that animaginary line connecting points of intersection of said first andsecond composite coils and an imaginary line connecting points ofintersection of said third and fourth composite coil are at right anglesand intersect at their respective centers, said third and fourthcomposite coils each consisting of two coils sharing one coil reel buthaving different numbers of coil turns, the coil turn ratio therebetweenbeing such as to form a nearly circular rotating magnetic field on thesurface part of test materials when the test material is inserted intothe magnetizing coils, and said third and fourth composite coils beingcoupled to said alternating current source means.

6. The apparatus of claim 5 wherein the angles respectively formed bythe intersection of the planes of the first and second composite coilsand the third and fourth composite coils in the direction in which thetest material is inserted are acute.

7. The apparatus claimed in claim 5, wherein alternating currentsupplied respectively to said coils has a phase difference of and thesaid coil turn ratio is in the range between '1.5 and 2.5.

8. The apparatus claimed in claim 5, wherein the angle formed by theintersection respectively of the planes of the first composite coil andthe second composite coil and the third and fourth composite coils arebetween 30 and 60.

1. An omnidirectional magnetic particle flaw detecting apparatus,comprising a first set of magnetizing coils consisting of a first planarcomposite coil and a second planar composite coil the planes positionedto intersect each other, each said composite coil being composed of twocoils sharing one coil reel but having different numbers of coil turns,the coil turn ratio between the two coils of each coil composite beingso determined as to produce a nearly circular rotating magnetic field onthe surface of a test material when the test material is inserted intothe magnetizing coils, and an alternating current source means coupledto the two coils of each of said composite coils and supplyingalternating current respectively differing in phase relative to the twocoils, whereby surface flaws can be detected with the same sensitivityindependent of the flaw direction.
 2. The apparatus claimed in claim 1,wherein the angle formed by the intersection of the planes of the firstcomposite coil and the second composite coil in the direction in whichthe test material is inserted is acute.
 3. The apparatus claimed inclaim 1, wherein the alternating currents supplied to said compositecoils have a phase difference of 120.degree., and said coil turn ratiois between 1.5 and 2.5.
 4. The apparatus claimed in claim 3, wherein theangle formed by the intersection of the planes of the first compositecoil and the second composite coil is between 30.degree. and 60.degree..5. The device of claim 1, further comprising a second set of magnetizingcoils consisting of a third planar composite coil and a fourth planarcomposite coil, the planes of said third and fourth composite coilsbeing positioned to intersect each other and being positionedconcentrically with said first and second composite coils such that animaginary line connecting points of intersection of said first andsecond composite coils and an imaginary line connecting points ofintersection of said third and fourth composite coil are at right anglesand intersect at their respective centers, said third and fourthcomposite coils each consisting of two coils sharing one coil reel buthaving different numbers of coil turns, the coil turn ratio therebetweenbeing such as to form a nearly circular rotating magnetic field on thesurface part of test materials when the test material is inserted intothe magnetizing coils, and said third and fourth composite coils beingcoupled to said alternating current source means.
 6. The apparatus ofclaim 5 wherein the angles respectively formed by the intersection ofthe planes of the first and second composite coils and the third andfourth composite coils in the direction in which the test material isinserted are acute.
 7. The apparatus claimed in claim 5, whereinalternating current supplied respectively to said coils has a phasedifference of 120.degree.; and the said coil turn ratio is in the rangebetween 1.5 and 2.5.
 8. The apparatus claimed in claim 5, wherein theangle formed by the intersection respectively of the planes of the firstcomposite coil and the second composite coil and the third and fourthcomposite coils are between 30.degree. and 60.degree..